A tail structure which adjoins a fuselage section of an aircraft includes a fuselage, lifting surfaces located at both sides of the fuselage such as wings or pylons for supporting the engines of the aircraft or other supporting structures, and a bulkhead unit for pressure-tight sealing of the fuselage section.
The rear pressure bulkhead (RPB) is the element that closes the pressurized area of the aircraft by its very back. It separates the pressurized part of the fuselage which constitutes the cabin allocating the passengers and the non-pressurized rear cone where the empennage, and other lifting surfaces and systems are located.
On conventional curved RPB solutions, this element has a concave shape from the side of the pressurized zone and is substantially hemispherical, so as to provide optimum use of the material of the bulkhead, acting as a membrane, to absorb the load due to the positive pressure difference inside the cabin. This bulkhead configuration requires additional stiffeners and other reinforcements to support the skin and load in case of reverse pressure cases, when there is negative pressure difference in the cabin for which the concave shape is not effective. Also, additional reinforcements are added at cut outs needed to allow the passing of systems pipes and access doors. These reinforcement elements constitute a significant penalty weight on the curved rear pressure bulkhead.
On flat RPB solutions, the structure is not optimized to sustain positive cabin pressure loads as it is not able to work as a membrane, but this solution already includes stiffeners and reinforcement elements that are effective for both positive and negative pressure load cases. Also, these reinforcements on a flat bulkhead are effective to integrate system pipe and access doors. This makes the flat bulkhead solution still competitive compared to the curved one, depending on the importance of these additional requirements.
Conventionally, the pressure bulkhead separates an anterior section of another aft fuselage section, sections corresponding to the pressurized and unpressurized areas of the aircraft, respectively. With this configuration, mainly the structure of the bulkhead assembly should be able, in addition to absorbing pressure loads, to maintain the shape of the fuselage and provide adequate strength to the coupling between the front and rear sections of the fuselage so as suitable for cross stiffening the fuselage.
For this reason, the assembly structure of known curved pressure bulkheads incorporates a frame and splices following the contour of the fuselage and intercostals. These frame and intercostals also resist the membrane reactions that the curved bulkhead transmits to the fuselage cylindrical structure, when pressure is applied. On a flat bulkhead, there are no membrane loads, so the intercostals are not mandatory, and the contour frame can be replaced by a T shape profile, removing the penalty weight and assembly complexity of these elements.
Known fuselages comprise a plurality of frames, stringers and beams, which act as reinforcing members of the aircraft skin. In order to integrate lifting surfaces with fuselage structures, discontinuities are usually performed in the fuselage skin, to allow the passing and continuity of support structures of such lifting surfaces.
Lifting surfaces may be divided into two or three independent parts or can be continuous. When divided they comprise a central box located inside the fuselage and two lateral boxes located at both sides of the fuselage or, as an alternative, the lifting surfaces can be divided into two lateral boxes joined at the symmetry axis of the aircraft. Structural boxes comprise at least a front spar and a rear spar extending in the longitudinal direction of the torsion box, upper and lower skins and ribs extending in the transversal direction of the structural box.
Regarding pylons, a first known configuration may include a pylon extending between both engines and located at a centered position of the height of the cylindrical part of the fuselage. The central part of the pylon is introduced into the fuselage, therefore the configuration implies a discontinuity or, at least, a cut-out in the fuselage skin.
In the known configurations the lifting surfaces are clearly separated from the rear pressure bulkhead, or what is the same, they are located at different sections in the longitudinal axis of the aircraft.